Reprinted with permission from Modern Woodworking Magazine.
It is important to understand what causes a tool to dull, according to Dave Rankin, director of technical education at the Grinderman’s Association. Under normal operation of a cutting tool, the heat build up during its use is one of the major causes of life reduction. Reducing this heat can, and often does, increase tool life.
Balance is a major factor in the reduction of tool life,” Rankin adds. “At 6,000 rpm an imbalance of only 1-gram creates a 22-pound rotational force on the spindle. This means the tool is not running smoothly, and therefore, the knives are being abused.
Proper Tool Grinding
Rankin says that a moulder operator should also be concerned with the accuracy of the tool grind. If two knives are being used, both knives should be ground to within .0002 inches or 5 microns. With normal chip removal this allows the shorter knife to still cut about 94 percent of the material that the longest blade is cutting. If the accuracy is out by .001 inches or 25 microns, then the shorter knife is only removing about 68 percent of the material that the longer knife is removing. This causes the longer knife, the one producing the finish cut, to heat up more quickly. The proper use of hook angle in the cutter heads and the correct grind angles are also critical. The harder the wood, the more the wood must be scraped, and thus the angle must be lower. The softer the wood is to cut, the more the wood must be sliced, and thus the angle must be higher.
The angle at which the knife is ground must be kept in a range that ensures the tip will not be thin and brittle, according to Rob Duve, sales manager for SCM Group and former moulder instructor at the Robert C. Byrd Hardwood Technology Center (now the Wood Education and Research Center). “Most companies recommend 25 degrees,” Duve said. “I prefer 23 degrees.”
Duve adds, “If the angle is too high, the tip becomes thin which leaves it brittle and susceptible to overheating. If ground too low, the point where the back of the knife meets the ground face will come in contact with the work surface.” Known as “heeling” this causes the fibers of the wood to become compacted affecting the finish of the wood.
“Keep in mind that when running high speed moulders, the back clearance angle of the knives is cut higher (around 30 degrees) in order to prolong the knife life under jointed conditions,” said Duve.
Operators must be aware of side clearance angles as well, according to Duve. When grinding, the grinding wheel penetrates the steel at a 90-degree angle. Therefore the sides of the profile will rub on the workpiece as it passes. This is especially a problem with deep profiles. To avoid this, Duve suggests that a side clearance angle must be ground. Tilting the wheel to one side or another depending on the side of the profile to be ground should accomplish this. “If no side clearance angle is ground, it generates excess heat in the knife which allows it to degrade faster,” he said.
The grinding wheel plays a role in heat generation as well. “More knives are probably ruined by excessive heat generated during grinding than by any other method,” said Rick Earp, tooling sales manager, Michael Weinig, Inc. “Knives may be burned because the grinding wheel is too hard or too coarse. Remember, there is no all-purpose wheel, so use the correct wheel for your immediate needs.”
The type of wheel being used should determine the proper material removal rate and traverse speed, according to Earp. Removal of too much material at one time with the incorrect wheel creates knife burn. Burn may also occur if the grinding wheel speed is too fast. Wheels that are run at a higher speed act harder. Insufficient coolant at the point of grind is yet another reason knife burn may occur.
Material and Material Feed Rate Issues
If too little stock is removed, the chip generated is too small to effectively draw heat from the knife. “It is generally agreed upon by machine manufacturers that at least .020 inches must be removed to create a chip large enough to cool the knife effectively,” said Duve. If possible, all lumber to be milled should be pre-planed and ripped to size to ensure not only a stock size the machine can accommodate, but also to remove any dust, grit and foreign matter that might damage the knives.
The material being run through the moulder, and the feed rate at which it is run, contribute to the tool life in a major way, according to Rankin. If the material is run too slowly through the machine, the tools will overheat. If the material is run through the machine too fast, the finish quality will suffer.
“The consideration of the tool type versus the material being run is very important,” Rankin said. “For instance, MDF requires the use of either carbide or diamond tooling. The grades of carbide can and should be fine-tuned to the hardness of the MDF. In the case of solid wood, the concern is how the wood affects the tool. For example, hickory is very hard after it is dried. Carbide is a very good choice for hickory. Poplar, on the other hand, will normally show a better finish with high-speed steel even though carbide can be used. Glued stock is another area that must be considered when looking at the expected tool life. All steels will not work with glued stock. Many glues will require the use of carbide or a very hard steel knife stock.”
Proper Tool Maintenance Aids In Tool Life
An important segment of proper tool maintenance is the care and handling of tools and knives, according to Earp. “Handle tools with care, never allowing metal to metal contact. These tools have very sharp and very fragile cutting edges. Store tools in a manner that will not harm them or persons within the tool room,” he added.
Earp said that a factor commonly overlooked is the value of good machine operators. “An operator that lacks mechanical ability, skill and common sense can basically eliminate the built-in quality of a machine or tool. In contrast, it is also a proven fact that unskilled men with the proper tools can produce more than skilled men without the proper tools. Therefore, it can be stated that top quality operators, machines and tools do not actually cost money, but instead can save on expenses and produce more income,” he said. “By far, the majority of tools that are being used in woodworking today are run until they either start to create a bad finish or until the noise becomes to loud,” said Rankin. “In both of these cases, the tools are overdull. The best way to tell when a tool dulls is with an ampmeter. A dull tool requires more horsepower to pull the knife through the work piece.
A predicted increase in the amount of power can be established, said Rankin. Once the tool reaches that level, the tool should be ground. To push another 10 minutes of production with the tool can cause an increase of 20 or 30 minutes in the grinding process as well as reduce the overall life of the tool.
Reprinted with permission from Modern Woodworking Magazine. Free subscriptions are available by completing an on-line Subscription Form.
